In the molding of plastic articles, it has long been an objective to cause flow paths or runner channels, as they are called, of a distribution system for injection molding material from an infeed channel to each of a plurality of injection gates and mold cavities to be substantially equal. Equal length flow paths from the infeed channel to the mold cavities enable the most uniform injection pressure to be achieved. Also, they result in a substantially uniform temperature condition of the injection molding material at the injection gates.
Control of the flow of molten polymeric injection molding material in a distribution system which is connected to each of a plurality of cavities also is important to achieve uniform filling of the cavities. Uniform filling is an important factor in the production of complete molded articles having consistent properties. If a mold cavity is not filled uniformly with polymeric material, the resultant molded article may be in a stressed condition and the article may warp to relieve the stresses. Also, the resultant molded article may be lacking in optical clarity.
Injection molding distribution systems which are capable of providing a uniform flow of molten polymeric material to a plurality of mold cavities are referred to as balanced runner systems. The term runner identifies the solidified injection molding material in a channel of the distribution system. A balanced runner system is one in which the runner channels of the distribution system from an infeed channel to the cavities are the same length and cross-section. Balanced runner systems are discussed by J. S. Walker and E. R. Martin in "Injection Molding of Plastics" 2nd Ed. The Plastics Institute, Iliffe Books, London, England, especially at pages 99 and 108-109. See also U.S. Pat. Nos. 3,533,594 and 3,951,375.
A typical prior art balanced runner system is shown in previously mentioned U.S. Pat. No. 3,533,594. The infeed channel is located at the center of a crossbar with mold cavitites being disposed at ends of the crossbar legs which are the runner channels. This simple pattern is termed a four cavity layout. Such a layout may be extended with crossbars being disposed at each end of the original crossbar. In another arrangement which is referred to as an "H" pattern, injection molding material is flowed from an infeed channel along a runner channel to each of two side runner channels and then bidirectionally along each side runner channel to the cavities. These arrangements require substantial space to accommodate the runner channels. Further, these patterns cannot be extended easily to include an increased number of mold cavities which are needed to mold simultaneously a substantial number of articles.
Some of these hereinbefore described prior art balanced arrangements are hot runner systems in which heating elements internal to the mold maintain the injection molding material in the distribution system in a molten condition. Hot runner systems involve substantially higher initial and maintenance costs than those for non-heated systems. Also, because the plastic material is in a molten condition in the distribution systems for a longer time before reaching the cavities, it may become degraded. In non-heated systems, the injection molding material in the distribution system from the infeed channel to the cavities is removed after each molding cycle. This is referred to as a removable runner system. For these systems, heating arrangements in the mold press are not required and the cavities can be closer together than in a hot runner system.
There are disadvantages associated with prior art balanced runner systems. The runners typically have been relatively large, generating more scrap or material to be reground for reuse and requiring larger molds. Larger molds in turn require larger presses with associated delivery barrels, resulting in greater residence time for the injection molding material. This could cause degradation of the injection molding material.
The disadvantages of balanced runner systems have discouraged their use in the past. It is little wonder that manufacturing engineers have opted for an unbalanced non-heated system comprising a simple two column layout of cavities with a main runner channel disposed between the columns of cavities and with transverse branches feeding the cavities. However, the trend toward the molding of substantial quantities of miniature articles having relatively thin walls has mandated another look at balanced runner systems. Desirably, the sought-after arrangement is a non-heated one which uses smaller molds and presses, faster cycles, and which results in less scrap and less degradation of the plastic material.
Clearly, to make the balanced runner approach more palatable to the industry, new techniques must be devised to overcome the hereinbefore discussed disadvantages. The prior art appears to be devoid of such solutions for non-heated balanced runner systems which are capable of providing a substantial number of articles in each cycle.